1. Field of the Invention
The present invention relates in general to an automated system for non-destructive inspection of metal through the use of EMAT (electromagnetic acoustic transducer) technology and more specifically to the use of such systems to detect surface and sub-surface pencil pipe defects in sheet metal.
2. Description of the Related Art
A variety of industries use sheet metal which is produced typically at a factory from ingots processed by a rolling mill. Ingots are heated and rolled by the mill into a long flat sheet which is then wound into a coil at the end of the mill. Thereafter, the coil is removed from the mill and shipped to other sections of the factory where further treatment processes are performed. To make these other processes have fewer interruptions, the coil maybe unrolled and joined at one of its ends to the end of another coil by welding apparatus. Any number of coils may be welded together in such fashion, depending on the desired length of process run.
Internal inclusions such as non-metallic inclusions and surface defects that may be found in processed metal rolls cause severe problems in down stream applications. When metal is drawn or formed, inclusions can result in surface blemishes, weak spots or tears in the product. In high quality applications, surface scratches can render the ultimate product useless and rejectable. Considering today's market, it's important to find these facts as soon as possible in order to take corrective action or to reroute the product to a less critical application. The rolled metal line is one possible place to locate these defects and begin volume inspection.
The detection of internal (volumetric) defects in steel rolls or plate using ultrasonics has been widely reported. Ultrasonic inspection methods are capable of detecting extremely small volume flaws in strip steel, but are limited due to the need to maintain couplant between the transducer and the steel strip. At a minimum, this couplant requirement slows the test speeds considerably, can introduce errors into the test results, and, in many cases, prevents the test from being performed at all.
The strip steel industry has a particular manufacturing problem of non-metallic inclusions in their strip steel product for use in the automotive/appliance industry which is manifested after the rolling operation. The "pencil pipe" inclusion is not likely to be visible prior to cold rolling. A method or other forming processes of detecting these, and other, inclusions prior to downstream processes is needed that could be used for on-line detection of pencil pipe defects in strip steel before the strip is rolled or formed to final product.
The pencil pipe inclusions are thought to result from extraneous non-metallic material trapped in the ingot that finally gets rolled into a thin inclusion or delamination. The defect typically occurs close to the edge of the strip (within about 25" of the edge).
In an ultrasonic technique, defects are detected through their interference with ultrasonic waves which are generated and detected by the interaction between a static magnetic field (B.sub.o) and eddy currents (g) induced by a high frequency magnetic field (B.sub.o).
This interaction allows the converting electromagnetic energy to mechanical energy and gives rise to two basic forces respectively known as:
F.sub.L =the LORENTZ force due to the interaction between free electrons and crystal lattice. PA1 F.sub.M =the magnetostrictive force due to local magnetization and its related striation. PA1 guided waves including plate waves, and PA1 bulk waves or free waves. PA1 1. Continuous operation in commercial, heavy industrial steel coil production mills, including pickling and final finishing lines. PA1 2. Inspection of a full range of pickled hot rolled steel strip grades including regular, low, and ultra low carbon, and high strength low alloy. PA1 3. Inspection of material gauge from about 0.050" to 0.200" in thickness. PA1 4. Inspection of material from about 30 to 80 inches in width. PA1 5. Inspection of 100% of the steel strip in all three dimensions: down-web, cross-web (except about 1.5" from each edge assumed to be trimmed later), and through-thickness. PA1 6. Operation on strip moving at line speeds up to about 1,200 feet per minute, accelerating up to about 250fpm/sec, and decelerating up to about 275 fpm/sec.
In the case of ferromagnetic materials (such as carbon steels) F.sub.M is about 10 to 100 times higher than F.sub.L.
In case of non-ferromagnetic materials (such as austenitic steels), F.sub.M is not present and F.sub.L only can be used for generation and detection.
Both Lorentz and magnetostrictive forces generate ultrasonic waves which are elastic waves that consist of oscillations of the crystal lattice of the metal around its equilibrium position. These ultrasonic waves are of two types, respectively:
Plate waves more particularly Lamb waves are preferred because they move as guided waves through the material, which results in a much higher detectivity.
In known detection systems such as a EUROPA system, each pole of the magnet is located under the strip and ultrasonic waves are merely produced across the width direction while the strip is moving.
These Lorentz and magnetostriction effects are reversible. This means that ultrasounds will interact with magnetic field, by producing electromagnetic waves which can be detected by coil-type receivers similar to those used as emitters.
By a suitable design of the transducers and of the magnetic field, Lamb waves are efficiently generated and consist of an optimal combination of longitudinal and transverse waves.
Two types of excitation modes can be used respectively s=symmetrical and a=non-symmetrical. These modes are characterized by the evolution of the virtual reflection factor across the thickness. Surface defects and internal defects will be detected depending on the selected mode and on the selected angle of incidence for a given strip thickness.
The symmetrical mode (s) is usually preferred because it is faster due to its higher group velocity.
Six modes of Lamb waves are used in the EUROPA system, i.e , S.sub.0, S.sub.1, S.sub.2 and A.sub.0, A.sub.1, A.sub.2. Depending on where the pointing vector (which is indicating the energy flow or the power flux density) will reach a maximum, surface or internal defects will be identified.
Electromagnetic acoustic transducers (EMATs) are an excellent method of producing Lamb waves since they require no fluid couplant. In addition, the wavelength of Lamb waves produced by EMATs is determined by the separation of adjacent conductors and the frequency can be easily changed by a software command. EMATs have been used for all types of inspections involving thin wall components of various shapes and defects in welds. EMAT Lamb waves are particularly suited to high speed automated inspections because there are no couplant limitations.
The prior art does not disclose or suggest how to select a Lamb wave mode that would be used for on-line detection of pencil pipe defects in strip steel before the strip is rolled to its final thickness. The pencil pipe inclusions are thought to result from extraneous non-metallic material trapped in the ingot that finally gets rolled into a thin inclusion or delamination. The defect normally occurs close to the edge of the strip (within 25" of the edge).
Thus there still is a need for an on line system for detecting of pencil pipe defects in strip steel using EMAT and optimized Lamb waves.